This invention relates to measuring and testing of pitting corrosion and it relates particularly to the instruments and electrochemical techniques used in monitoring pitting corrosion.
Two types of metallic corrosion are prevalent in aqueous systems. They are known as general and pitting corrosion. General corrosion proceeds uniformly over the metal surface at a relatively low rate. General corrosion rates are readily measurable with such methods as described in U.S. Pat. No. 3,687,610 to Gilson et al (Weight Loss) and U.S. Pat. No. 3,406,101 to Kilpatrick (Linear Polarization). Instrumentation used for linear polarization is like that described in U.S. Pat. No. 3,661,751 to Wilson.
Pitting corrosion proceeds 10-1000 times faster than general corrosion and occurs at small discrete areas rather than continuously over the metal surface. Because pitting occurs discontinuously on the metal surface, neither weight loss nor linear polarization can be utilized to measure its rate. Special methods of measurement need be employed.
Two such methods are physical measurement and non-linear polarization. Physical measurement involves determining the pit depth in mils (1,000 mils=1 inch) and dividing this depth by the total time (in years) during which the metal was exposed to the corrosive fluid. This gives the pitting rate in mils penetration (of the metal) per year. This method has the inherent disadvantage of being an "after-the-fact" type of determination. Measurement cannot be made in advance of the corrosion in order that corrective action can be taken. Also the process under surveillance for pitting corrosion must be stopped in order to make this measurement.
A more desirable method of pitting rate determination is non-linear polarization. With this method, pitting rates can be measured "as-they-happen" rather than "after-the-fact". Thus, corrective action can be taken to avoid premature equipment failure by a pit which has finally penetrated the metal. Non-linear polarization has the disadvantage of being time consuming. The person utilizing the method must also have some expertise of corrosion phenomena as it relates to non-linear polarization. In addition, specialized equipment is required.
In order to adequately describe our invention we need to discuss the non-linear polarization method in more detail.
Three or four pieces of an appropriate metal, hereafter referred to as electrodes, are attached to a holder in the manner described in the above mentioned U.S. Pat. No. 3,406,101 to Kilpatrick, U.S. Pat. No. 3,558,462 to Wilson, U.S. Patent No. 3,639,876 to Wilson and U.S. Pat. No. 3,632,495 to Watson et al. The electrodes (when four are used) are designated as reference, anodic test, cathodic test and auxiliary electrodes. At least the test electrodes are made of the metal whose properties in respect to pitting corrosion are to be monitored. The electrodes are inserted into the corrosive fluid for the pitting measurement and the test electrodes brought into equilibrium with the fluid.
The method is as follows:
The cathodic test electrode is increasingly polarized cathodically with respect to the probe body or the reference electrode until the applied potential is approximately -300 millivolts from the rest potential. As the potential is increased, the current which is required to be passed between the auxiliary electrode and the test electrode in order to maintain the potential between the latter and the probe body or reference electrode is recorded as a function of the applied potential in the form of a semi-log plot of current versus potential. This is referred to as the cathodic curve. After the cathodic curve is generated, the anodic test electrode is increasingly polarized anodically in a similar manner to the polarization of the cathodic test electrode. When the current passed between the test electrode and the auxiliary electrode is 50 milliamps, the potential is driven back towards the rest potential until 0 milliamps current is passed between these electrodes. As with the cathodic polarization, the anodic polarizations are plotted against current. This gives forward and reverse anodic curves. The intersection of the reverse anodic curve with the cathodic curve or its extrapolation yields the pitting current. The pitting current is converted into pitting rate (mils penetration per year), e.g., by using the following equation for standard size (9 cm.sup.2) electrodes: mils penetration per year=(1000).times.(Pitting Current).
Further discussion of this technique and a comparison of pitting rates obtained in this manner with actual rates can be found in the following publications:
R. L. Martin, CORROSION/77, (International Corrosion Forum, Mar. 14-18, 1977, San Francisco, California), Paper No. 140 PA1 W. S. Tait, CORROSION, Vol. 34, No. 6, pp. 214-218 (1978) PA1 R. L. Martin, MATERIALS PERFORMANCE, Vol. 18, No. 3, pp. 41-50 (1979)
The non-linear polarization can be performed in discrete steps or continuously.
It is an object of this invention to provide an instrument capable of determining pitting rates automatically and repetitively without the need to plot the anodic and cathodic curves. It is a further object of this invention to provide such an instrument which requires no corrosion expertise in order to obtain pitting rates with it. Other objects of the invention will become apparent from the following description.